U.S. patent application number 10/738599 was filed with the patent office on 2005-06-23 for cavity enhanced optical detector.
Invention is credited to Katchanov, Alexander, Paldus, Barbara, Provencal, Robert.
Application Number | 20050134836 10/738599 |
Document ID | / |
Family ID | 34677417 |
Filed Date | 2005-06-23 |
United States Patent
Application |
20050134836 |
Kind Code |
A1 |
Paldus, Barbara ; et
al. |
June 23, 2005 |
Cavity enhanced optical detector
Abstract
A cavity enhanced optical detector comprising: i) a source of
continuous wave laser light; ii) a high finesse resonant cavity
comprising at least three spaced apart, high-reflectivity mirrors
positioned to receive light from the laser light source; iii) at
least one photo-detector for measuring the extinction coefficient
of an analyte that is positioned in the resonant cavity; and iv)
one or more additional photo-detectors for measuring the intensity
of fluorescence emission and/or Raman scattering from the
analyte.
Inventors: |
Paldus, Barbara; (Sunnyvale,
CA) ; Provencal, Robert; (San Jose, CA) ;
Katchanov, Alexander; (Sunnyvale, CA) |
Correspondence
Address: |
Herbert Burkard
480 Oakmead Parkway
Sunnyvale
CA
94085
US
|
Family ID: |
34677417 |
Appl. No.: |
10/738599 |
Filed: |
December 17, 2003 |
Current U.S.
Class: |
356/73 ; 356/318;
356/432 |
Current CPC
Class: |
G01J 3/42 20130101; G01N
21/658 20130101; G01J 3/10 20130101; G01N 21/6402 20130101; G01J
3/44 20130101; G01N 21/39 20130101; G01N 2021/655 20130101; G01N
2021/651 20130101 |
Class at
Publication: |
356/073 ;
356/432; 356/318 |
International
Class: |
G01N 021/00; G01N
021/64 |
Claims
1. A cavity enhanced optical detector comprising: i) a source of CW
laser light; ii) a high finesse resonant cavity comprising at least
three spaced apart, high-reflectivity mirrors, to receive light
from the laser light source; iii) at least one photo-detector for
measuring an extinction coefficient of an analyte that is
positioned in the resonant cavity; and iv) at least a second
photo-detector for measuring intensity of fluorescence emission
from the analyte.
2. A cavity enhanced optical detector in accordance with claim 1,
further comprising at least a second photo-detector for measuring a
light scattering coefficient of said analyte.
3. A cavity enhanced optical detector in accordance with claim 1,
wherein said resonant cavity comprises four mirrors in a bow-tie
configuration.
4. A cavity enhanced optical detector in accordance with claim 3,
wherein a first and second of said mirrors are planar and a third
and fourth of said mirrors are plano-concave.
5. A cavity enhanced optical detector in accordance with claim 3,
wherein all four of said mirrors are plano-concave.
6. A cavity enhanced optical detector in accordance with claim 1,
configured as a cavity ring-down spectrometer.
7. A cavity enhanced optical detector in accordance with claim 1,
wherein said extinction coefficient and said intensity of said
analyte are measured substantially simultaneously.
8. A cavity enhanced optical detector in accordance with claim 1,
further comprising detection means for measuring Raman scattering
by said analyte of light provided by said source.
9. A cavity enhanced optical detector in accordance with claim 8,
wherein said detection means comprises a CCD array.
10. A cavity enhanced optical detector in accordance with claim 1,
wherein said analyte is contained in a container having a volume
smaller than a volume of said cavity.
11. A cavity enhanced optical detector in accordance with claim 10,
wherein said container is a Brewster Cell.
12. A cavity enhanced optical detector in accordance with claim 1,
wherein said at least one detector for measuring said extinction
coefficient is a photo-diode.
13. A cavity enhanced optical detector in accordance with claim 1,
wherein said detector for measuring said intensity of said
fluorescence emission is at least one of a photo multiplier tube
and an avalanche photo-diode.
14. A cavity enhanced optical detector in accordance with claim 2,
wherein said detector for measuring said scattering coefficient is
at least one of a photo multiplier tube and an avalanche
photo-diode.
15. A cavity enhanced optical detector in accordance with claim 2,
wherein said at least one detector for measuring said fluorescence
emission is a first plurality of detectors, and said at least said
second detector for measuring said scattering coefficient is a
second plurality of detectors.
16. A cavity enhanced optical detector in accordance with claim 2,
wherein said at least said second detector for measuring said
scattering coefficient of said analyte is a plurality of detectors,
wherein at least a first of the plurality of the detectors is
positioned to measure forward scattering and at least a second of
the plurality of detectors is positioned to measure back
scattering
17. A cavity enhanced optical detector in accordance with claim 1,
wherein said photo-detector for measuring said intensity of said
fluorescence emission further comprises a fixed frequency
filter.
18. A cavity enhanced optical detector in accordance with claim 1,
wherein said photo-detector for measuring said intensity of said
fluorescence emission further comprises a filter having a pass
frequency that can be varied.
19. A cavity enhanced optical detector in accordance with claim 1,
further comprising a dispersive element and detector array for
measuring said intensity of said fluorescence emission.
20. A cavity enhanced optical detector comprising: i) a source of
CW laser light; ii) a high finesse resonant cavity comprising at
least three spaced apart, high-reflectivity mirrors, to receive
light from the laser light source; iii) at least one photo-detector
for measuring an extinction coefficient of an analyte that is
positioned in the resonant cavity; iv) at least a second
photo-detector for measuring intensity of Raman scattering from the
analyte.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a cavity enhanced optical detector
which is able to substantially simultaneously measure extinction
coefficient, scattering and fluorescence and/or Raman scattering of
gaseous aerosols.
BACKGROUND OF THE INVENTION
[0002] The presence of various types of contaminants in the earth's
atmosphere or in the internal atmosphere of a work place or other
structure is a matter of world-wide concern. A great variety of
adverse effects including global warming, transmission of
contagious diseases and numerous long term health problems such as
emphysema have been attributed to a wide variety of atmospheric
contaminants. Atmospheric contaminants can be broadly divided into
three categories based on their physical form, i.e:, solid, liquid,
or gaseous and then in some of these cases further subdivided into
organic, inorganic and biological. For example, harmful inorganic
vapors present in the atmosphere include SO.sub.2, nitrogen oxides
and mercury. Inorganic solid contaminants include cement dust and
ammonium sulfate. Examples of organic vapors include
chlorofluorocarbons and various aromatic hydrocarbons. Particularly
at lower temperatures, many even relatively low molecular weight
organic compounds can be present in the atmosphere in liquid or
even solid particulate form. Of particular concern are atmospheric
liquid and solid biological materials ("bioaerosols")
[0003] The Bioaerosol Problem
[0004] Bioaerosols are airborne particles consisting of, or derived
from living organisms such as e.g., bacteria, viruses, molds,
fungi, pollens, dust mites insect remains and pet dander. They have
both natural and anthropogenic sources and are ubiquitous in the
earth's tropospheric boundary layer. Bioaerosols are found, for
example, in the workplace, in houses, in medical facilities, in
manufacturing operations, in dairy or other animal housing
facilities, in sites of sludge application, in recycling and
composting plants, in sanitary landfills, and in sewage plants.
Unlike most common non-biological origin atmospheric aerosols,
airborne bioaerosols can cause immediate disease, allergic
reactions and/or respiratory problems. Bioaerosols are also
particularly feared as potential biowarfare and terrorist
agents.
[0005] Current methods that measure aerosol particle size
distribution in real time provide insufficient information about
particle types (i.e., inorganic vs. organic vs. biological) and are
not able to identify specific microorganisms. Although efforts to
develop field useable instruments for the detection and
identification of airborne biological particles have accelerated
during the last several years, improved methods for characterizing
all types of aerosols, particularly bioaerosols, are urgently
needed. There is a need to quantify airborne microorganisms for
among other things: (i) the biotechnology industry, (ii) the
evaluation of indoor and outdoor air quality, (iii) investigations
of infectious disease outbreaks, and (iv) agricultural health
investigations. Today, there is a major technological void in
bioaerosol sampling techniques. To determine the sources and
effects of bioaerosols on human and animal health, a need exists
for an instrument that is capable of accurately and reliably
measuring aerosol optical properties while simultaneously
discriminating between biological and non-biological aerosols.
[0006] Biowarfare and Bioterrorism
[0007] Bioaerosols represent a particularly dangerous class of
biological weapons. As such, methods of detecting and
characterizing bioaerosols are greatly needed. As already
indicated, there currently does not exist any accurate real-time
method for the detection and identification of atmospheric and/or
indoor air biological particles. Present real-time aerosol
detection methods provide only a general indication of
size-distribution and provide virtually no information on particle
type.
[0008] Considering the enormous threat posed by bioaerosols to the
population at large, fieldable methods for detecting and
characterizing these hazards are urgently needed. Instruments that
operate in the field must be able to withstand mechanical vibration
and shock, produce accurate and reliable results without the need
for manual calibration or expert attendance, and preferably operate
without consumables.
[0009] Aerosols and Climate Change
[0010] In recent decades global warming has taken terrestrial
temperatures to their highest levels in at least the past
millennium. While the causes of global warming continue to be
debated, there is considerable evidence that atmospheric aerosols
play an important role. Because most aerosols reflect sunlight back
into space, they also have a "direct" cooling effect by reducing
the amount of solar radiation that reaches the surface of the
Earth. The magnitude of this reverse radiation (known as
"radioactive forcing") depends on the size and composition of the
aerosol particles, as well as their optical properties. It is
thought that aerosol cooling may at least partially offset the
expected global warming that is attributed to increases in the
amount of atmospheric carbon dioxide resulting from human activity.
The size of these various effects, is, however, not well
understood, mainly due to the lack of accurate optical data. An
International Panel on Climate Change has identified radiative
forcing due to the presence of aerosols as one of the most
uncertain components of climate change models and as a topic
urgently in need of further research.
[0011] There is therefore a pressing need for an analytical
instrument having the following capabilities: i) the ability to
substantially simultaneously detect the presence in the atmosphere
of inorganic, organic and bioaerosols; and ii) distinguish
bioaerosols from non-biological particulates and dispersed liquids
(organic and/or inorganic) and also to identify certain specific
bioaerosols. To accomplish these multiple tasks the instrument
should be able to do the following: 1) perform extinction and
Rayleigh scattering measurements to estimate the quantity and size
distribution of liquid and solid particles (i.e., inorganic,
organic and biological) present in the atmospheric sample: and 2)
measure laser induced fluorescence and/or Raman scattering of
bioaerosols to facilitate identification. It should be noted that
although fluorescence can be induced in certain organic compounds,
such fluorescence is normally much weaker than that of biological
compounds and also the fluorescence spectra of organic and
biological materials are significantly different. The difference
between the quantity of aerosol indicated by procedures 1 and 2 is
therefore also a measure of the inorganic and organic
(non-biological) liquid and solid atmospheric contaminants.
Accuracy is, of course, enhanced if measurements 1 and 2 are
performed substantially simultaneously. In addition, if possible,
the instrument should be able to perform Rayleigh scattering and
fluorescence at several wavelengths to permit estimation of
particle size distribution. Likewise, the Raman spectrum of many
compounds are unique and frequently specific organic or biological
components can be detected by analyzing the Raman scattering of
complex mixtures including aerosols. The distinction between Raman
scattering and Rayleigh scattering is that the latter is at the
same wavelength as the incident radiation while Raman scattering is
at a longer wavelength (Stokes) or higher frequency (anti-Stokes).
A reference to "scattering" herein, without further qualification,
is to be construed as Rayleigh scattering.
[0012] Designing an instrument that can accurately and rapidly
perform these multiple measurements is technically challenging. In
particular, building an apparatus that exploits the intrinsic
fluorescence and/or Raman emissions of bioaerosol particles for
their detection and classification is challenging for several
reasons. First, the particles of interest may be present in very
low concentration in a dominant background. Average fluorescence or
Raman spectra accumulated for a population of aerosol particles
will frequently yield little or no information about the few
particles of specific interest, i.e., single-particle spectra are
required. Second, fluorescence signals are generally weak because
individual particles generally contain only a few picograms of
material. Additionally, only a small fraction of the mass of most
biological particles consists of fluorophores. Likewise, Raman
scattering by its nature is weaker than the illuminating radiation.
Third, particles are generally dispersed nonuniformly in the air
(their concentration fluctuations follow the Kolmogorov spectrum of
atmospheric turbulence), and they must be detected at random times
as they are carried by a stream of air through an optical cell.
Fourth, an optimal detector must be able to excite particles in the
ultraviolet where most biological molecules (and hence biological
particles) fluoresce to a significant extent. Ultraviolet laser
sources are generally costly and have a relatively low energy
output. In many environments the dominant background particles are
nonbiological so that a minority concentration of bioaerosols must
be differentiated from these nonbiological particles. However,
because the UV-excited fluorescence is typically weaker from
non-biological as compared to biological particles, the
un-dispersed fluorescence intensity can be used to differentiate
between these two types of particles. In the case of Raman
spectroscopy, most biological particles show their strongest
emission when excited by radiation in the visible and near UV
region (100-700 nm), so an optimal detection instrument must be
able to provide incident radiation in this wavelength range.
[0013] We have developed an instrument using the principles of
cavity enhanced optical detection which is capable of performing
the above-indicated multiple measurements i.e., extinction
coefficient, scattering and fluorescence and/or Raman scattering.
In addition, our preferred instrument is uniquely compact and
portable, and has low power consumption. These features are highly
advantageous since much monitoring of atmospheric pollutants must
take place by aircraft at altitudes of up to 50,000 feet.
[0014] The operation of our instrument, which is also readily
integrated into an indoor air quality monitoring system, involves
the following steps:
[0015] 1) Determine the extinction coefficient by measuring the
ring down time using filtered air (i.e., an atmospheric sample free
from any liquid or solid particles). This will provide the
"background" extinction coefficient of the instrument (i.e., any
absorption due solely to gases present in the air and also any
scattering or absorption due to the instrument mirrors and
intra-cavity interfaces).
[0016] 2) Measure the extinction coefficient of the aerosol
containing (unfiltered) atmospheric sample. The difference between
results 1 and 2 provides the extinction due to the aerosols present
in the sample. Although not essential, simultaneous measurement of
scattering and extinction provides enhanced accuracy.
[0017] 3) Measure scattering since extinction minus scattering
equals the absorption by the liquid and/or solid aerosol
particles.
[0018] 4) Measure scattering simultaneously at several different
wavelengths. This provides particle size information since
scattering is a function of particle size and is proportional to
1/.lambda..sup.4
[0019] 5) Obtain the fluorescent and/or Raman spectra to permit
differentiation between biological and non-biological samples.
Since the strength of the fluorescence of biological particles is
significantly greater than that of organic and inorganic particles,
by appropriate choice of the incident laser wavelength ( near UV of
100 to 400 nm, or visible 400-700 nm, is particularly suitable) one
can virtually eliminate any significant fluorescence by the
non-biological constituents of the sample. In addition, a plot of
the fluorescence spectra permits identification of certain
bioaerosol components. For Raman detection incident radiation in
the 400 to 700 nm range (visible) is especially suitable.
[0020] As previously indicated, the extinction coefficient of the
aerosol containing sample is a measure of the combined effects of
scattering by the cavity mirrors and the liquid and solid
particulate content of the sample plus absorption by the solid,
liquid and gaseous sample components. If scattering by the sample
alone is then determined, the difference between extinction and
scattering is equal to absorption by the aerosol constituents
present. Finally, fluorescence or Raman measurement permits
estimation of the biological component of the total liquid and
solid particulates, which can frequently be identified by their
Raman spectrum.
[0021] Optical detection is the determination of the presence
and/or concentration of one or more target species present in a
sample by illuminating the sample with optical radiation and
measuring optical absorption, induced fluorescence, and/or optical
scattering by the sample. Optical detection has a wide variety of
applications, and a correspondingly wide variety of optical
detection methods are known. Cavity enhanced optical detection
entails the use of a passive optical resonator, also referred to as
a cavity, to improve the performance of an optical detector. Cavity
enhanced absorption spectroscopy (CEAS), sometimes called
integrated cavity output spectroscopy (ICOS), and cavity ring down
spectroscopy (CRDS) comprise the most widely used cavity enhanced
optical detection technology. Cavity enhanced optical detection,
either CEAS or CRDS can be used for solid, liquid, aerosol, or
gaseous samples.
[0022] The intensity of single-mode radiation trapped within a
passive optical resonator decays exponentially over time, with a
time constant .tau., which is often referred to as the ring-down
time. In practice, it is desirable to ensure that only a single
resonator mode has an appreciable amplitude, since excitation of
multiple resonator modes leads to multi-exponential radiation
intensity decay (i.e., multiple time constants), which
significantly complicates the interpretation of measurement
results. The ring-down time .tau. depends on the cavity round trip
length and on the total round-trip optical loss within the cavity,
including loss due to absorption and/or scattering by one or more
target species within a sample positioned inside the cavity. Thus,
measurement of the ring-down time of an optical resonator
containing a target species provides spectroscopic information on
the target species. Both CRDS and CEAS are based on such a
measurement of .tau..
[0023] In CRDS, an optical source is usually coupled to the
resonator in a mode-matched manner, so that the radiation trapped
within the resonator is substantially in a single spatial mode. The
coupling between the source and the resonator is then interrupted
(e.g., by blocking the source radiation, or by altering the
spectral overlap between the source radiation and the excited
resonator mode). A detector typically is positioned to receive a
portion of the radiation leaking from the resonator, which decays
in time exponentially with a time constant .tau.. The
time-dependent signal from this detector is processed to determine
.tau. (e.g., by sampling the detector signal and applying a
suitable curve-fitting method to a decaying portion of the sampled
signal).
[0024] Single spatial mode excitation of the resonator is also
usually employed in CEAS, (sometimes called integrated cavity
output spectroscopy (ICOS)), but CEAS differs from CRDS in that the
wavelength of the source is swept (i.e., varied over time), so that
the source wavelength coincides briefly with the resonant
wavelengths of a succession of resonator modes. A detector is
positioned to receive radiation leaking from the resonator, and the
signal from the detector is integrated for a time comparable to the
time it takes the source wavelength to scan across a sample
resonator mode of interest. The resulting detector signal is
proportional to .tau., so the variation of this signal with source
wavelength provides spectral information on the sample. Note that
CEAS entails a relative measurement of .tau..
[0025] In cavity enhanced optical detection, the measured ring-down
time depends on the total round trip loss within the optical
resonator. Absorption and/or scattering by target species within
the cavity normally accounts for the major portion of the total
round trip loss, while parasitic loss (e.g., mirror losses and
reflections from intracavity interfaces) accounts for the remainder
of the total round trip loss. The sensitivity of cavity enhanced
optical detection improves as the parasitic loss is decreased,
since the total round trip loss depends more sensitively on the
target species concentration as the parasitic loss is decreased.
Accordingly, both the use of mirrors with very low loss (i.e., a
reflectivity greater than 99.99 per cent), and the minimization of
intracavity interface reflections are important for cavity enhanced
optical detection.
[0026] Several recently published treatises, including the
references cited therein, cover most currently reported aspects of
CRDS and CEAS technology: "Cavity-Ringdown Spectroscopy" by K. W.
Busch and M. A. Busch, ACS Symposium Series No. 720, 1999 ISBN
0-8426-3600-3 and "Cavity Enhanced Spectroscopy" R. Peeters,
Katholieke Univeristeit Nijmegen, The Netherlands, 2001, ISBN
90-9017628-8. CEAS is also discussed in a recent article entitled
"Incoherent Broad-band Cavity-enhanced Absorption Spectroscopy by
S. Fiedler, A. Hese and A, Ruth Chemical Physics Letters 371 (2003)
284-294. However, none of these references teaches the simultaneous
measurement of Rayleigh scattering, extinction coefficient and
fluorescence and/or Raman scattering.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a schematic of a two mirror (numbered 1 and 2)
CRDS system. In the basic CRDS technique, a laser beam is injected
into an optical cavity formed by two (or more) high-reflectivity
mirrors, filling it with photons. The photons bounce back and forth
between the mirrors up to 15,000 times. Most of them stay within
the cavity for about 10 microseconds, traveling several kilometers
during this time. In each round trip, about 0.01% of the photons
pass through mirror 2 and strike a photo-detector 3. At a time
t.sub.0 the laser beam shuts off, and all the photons escape from
the cavity over the next few microseconds.
[0028] FIG. 2 shows a typical ringdown curve, which plots the
intensity of light inside the cavity as a function of time (in
microseconds) after the laser shuts off. The intensity of light
inside the cavity decreases exponentially with a time constant
.tau.(normally about 10 microseconds), the time interval over which
the intensity of light diminishes by 1/e. The equation set forth
within FIG. 2 above indicates that the intensity of light
(I.sub.circ) circulating inside the cavity at any given moment
equals the intensity of light which was circulating in the cavity
at t.sub.0, when the laser beam shut off, multiplied by e to the
-t/.tau..
[0029] This mathematical relationship holds not only for the light
circulating within the cavity, but also for the light escaping from
the cavity, since these two quantities are related by the
transmissivity of the mirror 2, which is a constant. The light
escaping from the cavity strikes the photodetector 3, which
responds with a voltage proportional to the intensity of the
incident light. The voltage across the detector as shown in FIG. 1
therefore decays exponentially starting at t.sub.0, when the laser
beam shuts off. The voltage across the photodetector at time t
equals the voltage across the photodetector at t.sub.0 multiplied
by e to the -t/.tau..
[0030] The fundamental principles of operation of our CRDS (or
CEAS) instrument are the same as for a two mirror cavity, The laser
light is allowed to build up in the resonator, and when it reaches
a threshold, it is shut off (e.g., with an external modulator).
This trigger initiates the acquisition of the ring-down decay
constant as well as the measurement of scatter, which will also
exponentially decay as the intracavity intensity exponentially
decays. By using multiple laser input wavelengths, scattering and
fluorescence at multiple wavelengths can be detected
simultaneously. One can detect the fluorescence intensity or
integrated fluorescence signal which will behave similarly to the
scattering signal but will, of course, be at a wavelength longer
than the original laser excitation wavelength.
[0031] The presence of aerosol particles will increase the cavity's
decay rate, due to both scattering and absorption by the particles.
By measuring the decay rate of the cavity, a CRDS or CEAS
instrument can determine the extinction coefficient of the aerosol
containing sample gas with extreme sensitivity. Moreover, high
optical intensities build up within the ringdown cavity. This
enhances the intensity of both the scattering (Rayleigh and Raman)
and fluorescence signals from aerosols. If one uses a linear cavity
as shown in FIG. 1, the intracavity power circulates in both
directions along the cavity axis thereby forming a standing wave.
This wave can have maxima and minima where scattering is maximized
or minimized, leading to a periodic variation of the scattering
signal along the cavity especially for relatively small particles
(e.g., 0.1 um particle vs. 1.5 um light). Also this changes the
complexity of the actual scattering profile in that a single
particle can scatter light in both directions, which can confound
the scattering signatures from asymmetric particles. In a ring
cavity, the intracavity light is a traveling wave which is
unidirectional (i.e., circulates in one direction only, as shown in
FIG. 3) and thus does not have modes and antinodes. This simplifies
the scattering profiles. However, not all ring resonator designs
are equal. For example, the sample volume is preferably minimized
in order to enhance the response time of the detector. Also, there
must be enough space between the beam paths of the ring resonator
for the sample to only "see" one arm of the resonator (i.e., a
traveling wave going in only one direction). This is sometimes
difficult to achieve with a triangular resonator, and a square
resonator creates a large footprint. Therefore, although a three
mirror ring, or four mirror rectangular (including square) cavity
can be used to practice the invention, our preferred cavity
configuration is a "bow-tie" shape as shown in FIGS. 3-6.
[0032] FIG. 3 is a schematic top view of a CRDS instrument
according to the present invention utilizing a single
multi-wavelength (frequency doubled) laser.
[0033] FIG. 4 is a schematic top view of a CRDS instrument
according to the present invention using three different lasers
each of a different wavelength.
[0034] FIG. 5 is a three dimensional schematic of a portion of a
CRDS instrument according to the present invention showing the
ringdown mirrors, beam path and fluorescence and scattering
collection lenses.
[0035] FIG. 6 shows a cavity design according to the present
invention utilizing a Brewster cell to contain a liquid sample or a
"dirty" sample which could contaminate the cavity mirrors.
[0036] FIGS. 7a and 7b show a preferred design for the sample gas
inlet system.
[0037] FIG. 8 is a schematic which illustrates the flow pattern for
the sample gas and also for the purge gas which helps to maintain
the mirror surfaces clean and highly reflective when using our
preferred gas inlet system.
DETAILED DESCRIPTION OF THE DRAWINGS
[0038] Examples of CRDS instruments or portions thereof in
accordance with the present invention are shown in FIGS. 3-8. Note
that essentially the same configuration is equally applicable to a
CEAS instrument.
[0039] Although a three mirror triangular or four mirror
rectangular cavity can be utilized, our instrument preferably has a
unique four mirror "bow-tie" cavity configuration as shown in FIGS.
3-6. The cavity consists of a high finesse resonator using four
mirrors (preferably plano-concave) shown as 3.1, 3.2, 3.3 and 3.4.
This configuration has several unexpected advantages, as compared
with conventional two- and three-mirror designs, including: (i)
simplified alignment, (ii) longer optical interaction length, (iii)
uniformity of mirror optics; and (iv) simplified coupling of
multiple laser wavelengths into the cavity since when using
multiple lasers (as shown in FIG. 4) each wavelength can have its
own input coupling mirror. In addition, the bowtie configuration
increases the effective cavity path length, as compared to a
similar size linear cavity. As shown in FIG. 3, the laser beams
3.5, 3.6 and 3.7 strike all four mirrors, making two passes through
the cavity, i.e., four passes for one round-trip. When using all
plano-concave mirrors, all four mirrors can be identical and can,
therefore, be fabricated in a single coating run. The cost of a
mirror coating run is high, so this simplification in mirror optics
significantly reduces the CRDS system cost. As an alternative to
four plano-concave mirrors, one can use two flat and two
plano-concave mirrors. Again, although the mirror substrates would
not all be identical, a single coating run could be carried out. In
addition, if the input laser(s) do(es) not vary in frequency, only
one of the mirrors, e.g., 3.1 in FIG. 3, of the cavity needs to be
dithered to provide a resonant cavity. Alternatively, one can
utilize a fixed cavity and dither the laser. By using a high
finesse, optical build-up ring down cavity as shown, we have
developed a lower power, small, lightweight continuous wave (CW)
laser spectrometer that can be utilized for highly sensitive
scattering, fluorescence and Raman measurements. The capability
exists of exciting and detecting fluorescence from aerosols in our
ringdown cavity. We have found that it is possible to discriminate
between biological and non-biological aerosols based on their
fluorescence and/or Raman spectra. A shorter excitation wavelength,
will normally be absorbed by bioaerosols more efficiently (compared
to organic molecules) and thus produce stronger fluorescence
signals.
[0040] By way of example as shown in FIG. 3, we can determine
extinction coefficient, absorption and fluorescence using a single,
commercially available solid-state continuous wave (CW) 976 nm pump
laser. Because the 976 nm pump laser frequency can be doubled twice
to produce both 488 nm (blue visible) and also 244 nm (UV) light
using known frequency doubling techniques, (e.g., wave guides or
non-linear crystals) we can use this single 976 nm laser to excite
the fluorescence of bioaersols with 488 nm blue and/or 244 nm UV
light. Raman scattering can also be stimulated by the 488 nm light.
Alternatively, as shown in FIG. 4, our cavity design permits the
use of several independent CW lasers (4.1, 4.2 and 4.3) of
different wavelengths emitting into the cavity simultaneously
through different input mirrors It is not practical to measure
Rayleigh scattering, bioaerosol fluorescence and Raman scattering
using a single fixed wavelength laser. Also, as previously
indicated, use of a single wavelength laser precludes obtaining
meaningful particle size distribution information.
[0041] As indicated and shown in FIGS. 3, 4, 5 and 8, our cavity
ringdown cell preferably uses a four mirror bowtie cavity
configuration. This configuration provides a number of advantages.
First is ease of alignment. If desired, the four mirrors allows for
one, two or three input and likewise one, two or three output
mirrors. Therefore, no pre-cavity beam combining optics are
required, which greatly facilitates cavity alignment. Secondly,
this configuration inherently results in twice the optical path
length compared to a similarly dimensioned three mirror ring
cavity. Finally, as previously indicated, this configuration can
use four identical plano-concave mirrors, thus only one mirror
coating run is required. This results in a meaningful cost saving
in instrument production.
[0042] In one embodiment of this instrument, as shown in FIG. 4, it
contains a triple wavelength, four mirror ringdown cavity filled by
light from, for example, a 1.55 .mu.m infrared distributed feedback
(DFB) diode laser, a 675 nm visible Fabry-Perot (FP) diode laser
and a 275 nm frequency tripled Ti:sapphire (UV) laser. A Nd:YAG
pumped dye laser (red) can, if desired, be substituted for the
Fabry-Perot laser. A frequency doubled diode laser can be used to
provide light at 830 nm (near IR) and 415 nm (visible). Our
instrument is capable of measuring both extinction and scattering
at 675 nm using a photomultiplier tube. The measurement of both
scatter and extinction at 675 nm enables determination of the
single scatter albedo, .OMEGA., at this wavelength in accordance
with the formula below where .sigma..sub.scat is the scattering
coefficient and .sigma..sub.abs is the absorption coefficient: 1 =
scat scat + abs
[0043] The single scatter albedo is an important parameter which
indicates atmospheric visibility. In the embodiment illustrated in
FIG. 3, the visible laser source consists of a fixed frequency
diode laser emitting at 976 nm (near IR). Use of two doubling
crystals allows for the generation of both 244 nm (UV) and also 488
nm (blue visible) light. Suitable detectors for the ringdown
measurements are, by way of example, an InGaAs or Germanium
amplified photodetector for the 976 nm channel and silicon
photodiodes with a high gain preamplifier for the 244 nm and 488 nm
signals. Scattered photons and fluorescence emission can be
monitored using appropriate detectors placed above the ringdown
cell axis as shown in FIG. 5. Blue photons (488 nm) scattered by
particles in the beam path during the ringdown event can be
collected and monitored using, for example, a photomultiplier tube
(PMT) module. Scattered IR photons (976 nm) can be measured using
an avalanche photodiode together with a 976 nm narrow bandpass
filter to remove any longer wavelength fluorescence. Fluorescence
signals are collected and monitored using a PMT module and
appropriate notch filters to remove the 244, 488 and 976 nm
radiation. Fluorescence can also be measured using a diffraction
grating and CCD (charge couple device) array detector for complete
fluorescence spectrum acquisition. Ringdown, scattering, Raman and
fluorescence data are all acquired simultaneously. The measured
ringdowns are analyzed and the extinction coefficients can be
calculated using, for example, Windows based software. As shown in
both FIGS. 3 and 4, a series of high reflectivity planar
mirrors(3.8 and 3.9 in FIG. 3, and 4.4 through 4.9 in FIG. 4) are
used to direct the output from the source laser or lasers into the
ringdown cavity. If it is important to analyse the absorption
features of one or more aerosol components, a tunable input laser
capable of being tuned over the wavelength range of interest can be
used in lieu of individual fixed frequency lasers or a frequency
doubled laser. Optical parametric oscillator (OPO) lasers are
suitable where tunability is desired.
[0044] Using either multiple input lasers, or frequency doubling of
a single laser, our four mirror bow tie CRDS instrument shows a
number of unique advantages:
[0045] Ultra-high extinction sensitivity. Our instrument has the
ability to measure the extinction coefficient of an aerosol with a
sensitivity up to 10-.sup.-11 cm.sup.-1 Hz.sup.-1/2.
[0046] Ultra-high scattering sensitivity. By using high
reflectivity mirrors, (e.g., >99%) there is a significant
buildup of the optical intensity inside the cavity. This enhances
its sensitivity to scattered light. The sensitivity of our
instrument to scattered light is at least equal to that of the most
advanced particle counting detectors.
[0047] High fluorescence sensitivity. Particularly with our bow tie
ringdown cavity, the UV light which is preferably used for exciting
fluorescence can build up by a factor of 100 to 1000 thereby
providing intracavity power levels exceeding 1 watt with only a few
mW of incident laser light. Utilizing Brewster-cells within the
cavity as shown in FIG. 6, the sample can be confined to a small
volume to further increase signal strength. Alternatively in lieu
of UV light, fluorescence can be induced by visible light
(preferably in the 400-500 nm range). Therefore, at least one of
the laser beams used in the present invention should provide light
with a wavelength in the range of 100-500 nm. Our instrument can
measure aerosols with particle sizes ranging from 0.5 to 100
microns. It has an accuracy and precision of 1% for aerosols with
an extinction coefficient greater than 10.sup.-5 m.sup.-1, with a 1
second integration time.
[0048] Self-calibrating operation. Since CRDS is inherently
self-calibrating, our instrument requires no test samples or
consumables.
[0049] Inherent robustness. In CRDS, the measurement of the
extinction coefficient depends only on the rate of decay of light
inside the cavity. The measurement is therefore insensitive to
laser intensity fluctuations.
[0050] Design flexibility. Our basic platform can be easily
reconfigured to operate at a variety of different wavelengths and
can incorporate multiple wavelengths and/or tunable lasers in one
device. The instrument can be adapted to measure aerosols of
varying particle size and provide information about their size
distribution Since the instrument measures the fluorescence
emission from fluorophores interacting with the intracavity laser
radiation it is highly advantageous that the optical intensities
which build-up inside our high finesse cavity are able to generate
high levels of detectable fluorescence necessary to permit aerosol
particle type discrimination.
[0051] Detectors
[0052] Extinction, fluorescence and Rayleigh scattering can be
measured using known types of detectors such as avalanche
photo-diode or photo multiplier tube (PMT) detectors coupled with
appropriate collection lenses positioned as shown in FIG. 5. Charge
couple device (CCD) arrays are particularly useful for Raman
detection. Scattered photons and fluorescence emission can be
monitored using appropriate detectors placed above the ringdown
beam path. As seen in FIG. 5, the preferred detection system
comprises a plurality of forward/back Rayleigh scattering
collection lenses (5.2) and also fluorescence (and/or Rayleigh and
Raman scattering) collection lenses (5.3) place above and
advantageously also below the beam path (5.4) and the cavity
mirrors (5.1). Band pass filters are advantageously positioned
between each collection lens and its corresponding detector to
remove (block) extraneous frequencies. For example, the detector
used for measuring fluorescence should have a filter which blocks
the emission frequencies of the laser. It is important to note that
fluorescence and Rayleigh scattering detection normally requires at
least one different set comprising a collection lens, filter and
detector. However, it is possible to use a single unit comprising a
collection lens, detector and a tunable filter to detect both
fluorescence and Rayleigh scattering. Raman scattering is
preferably detected using a dispersive element (e.g., a diffraction
grating) and a CCD array detector. As sample flow traverses the
ringdown cell, transverse (perpendicular) to the optical axis, as
shown in FIG. 8, photons scattered by particulate matter are
collected using a multi-unit optical collection system placed above
(and/or below) the central portion of the ringdown cell as seen in
FIG. 5. An objective lens is employed to collect photons emitted
within the field of view of the objective lens. Photons scattered
by particles within this viewing region are collected by the lenses
and relayed to an appropriate detector. Although a single (or
multiple) scattering detector(s) located above the beam
intersection point (indicated as 3.0, 4.0 and 5.2 and 5.3 in FIGS.
3, 4 and 5, respectively, can be used, the use of multiple
detectors to measure both foreword and back scattering is
advantageous as a comparison of forward and back scattering
provides a basis for conclusions about the shape of the aerosol
particles. Suitable bandpass filters are used to remove interfering
fluorescence emission resulting from the visible or UV excitation.
An identical collection system can be used for fluorescence
detection. In this case, appropriate band pass filters can be used
to remove scattered photons. Fluorescence emission as well as
Rayleigh scattered 488 nm photons can be monitored using, for
example, a photomultiplier tube (PMT) module, while scattered IR
photons are preferably measured using a large area avalanche
photodiode. A diffraction grating and CCD array detector can be
incorporated to provide complete spectral acquisition in
real-time.
[0053] Brewster Cell
[0054] As shown in FIG. 6, our instrument can be adapted when it is
sometimes necessary to perform measurements on a liquid sample or a
highly contaminated gaseous sample The adaption consists of
containing the sample within a Brewster cell present within the
optical cavity. A detailed explanation of the use of a Brewster
cell in connection with CRDS or ICOS is set forth in co-pending,
commonly assigned U.S. patent application Ser. No. 10/700947, filed
Nov. 31, 2003, the teaching of which is incorporated herein by this
reference. As indicated, cavity enhanced optical detection can be
used for liquid, aerosol, or gaseous samples. For gaseous samples,
intracavity interfaces are typically not present, so there are no
corresponding interface reflection losses to contribute to round
trip parasitic loss. However, intracavity interfaces are typically
present for aerosol or liquid samples. For example, contamination
of the mirror surfaces by aerosols can create problems so that
these samples are advantageously enclosed in an intracavity cell.
This cell will create interfaces (e.g., windows) within the optical
resonator. Likewise, for a liquid sample contained in a flow cell
present within a cavity, the interfaces between the liquid and the
inner wall of the flow cell as well as the exterior wall surfaces
of the flow cell are all intracavity interfaces. U.S. Pat. No.
6,452,680 teaches the minimization of intracavity reflection loss
when examining solid or liquid samples by positioning the sample
such that optical radiation circulating within the optical
resonator is, insofar as possible, incident on the sample-induced
interfaces at an angle approximating Brewster's angle and is
p-polarized relative to these interfaces. Since reflection is
minimized for p-polarized incidence on an interface at Brewster's
angle, this arrangement significantly reduces reflection-induced
parasitic loss. FIG. 6 is a schematic illustration of this cell
configuration. In this design the liquid flow channel is tilted
within the cell so that the light beam strikes each surface at the
correct Brewster's angle for the specific interface (e.g.
air.fwdarw.fused silica.fwdarw.liquid or gaseous
sample.fwdarw.fused silica.fwdarw.air). With the appropriate
polarization of light, the interface reflections are minimized,
thereby allowing the light to pass back and forth through the cell
multiple times, resulting in a relatively long ring-down constant.
In the example shown in FIG. 1b, using a fused silica cell and
water as the sample liquid, angle e is 7.9.degree. and angle a is
55.6.degree. so that the light refracts through the cell, hitting
each interface surface at approximately Brewster's angle for
minimum reflection. In FIG. 6 solid line 1 indicates the light path
through flow cell 5 including flow channel 6 containing the analyte
sample. Dashed line 1.1 shows the altered light path if the
refractive index of the analyte changes. Note that mirrors 6.2 and
6.3 are planar so that light path 2 is not affected by the change
in refractive index. Mirrors 6.1 and 6.4 will preferably be
plano-concave as shown. Note that light paths 2, 3 and 4 are shown
as dotted lines since the flow cell is out of the plane of these
lights paths and only is intersected by beam path1 (or 1.1).
Extinction is measured by detector 6.5. Photodetectors 6.6, which
are also out of the plane of the beam path, are meant to represent
multiple detectors, as previously described, to measure Rayleigh
scattering, Raman scattering and fluorescence.
[0055] Air Flow
[0056] A preferred sample handling system consists of a stagnation
region which the sample gas enters vertically and then exits into
the cavity via horizontal flow channels. This sample inlet design,
as shown in FIGS. 7a, 7b and 8, results in a laminar flow of the
sample gas transverse (perpendicular) to the ringdown cavity
optical axis. As shown, the illustrated sample handling system
injects a laminar sample flow into the ringdown cavity and also
provides a protective purge gas (e.g., dry, filtered air or
nitrogen) flow on both sides of the sample region between the
cavity mirrors and the sample. Without use of the stagnation region
and the vertical entry of the sample gas into the stagnation region
it is sometimes difficult to achieve the laminar sample flow which
greatly facilitates accurate sample analysis.
[0057] Detecting Fluorescence in the Ringdown Cavity
[0058] Fluorescence spectroscopy is an ideal extension of CRDS due
to the inherent intracavity power build-up of high-finesse optical
cavities. The intracavity power can be estimated by: 2 I
intracavity = P c T
[0059] where PC is the power coupled into the cavity mode being
observed, .zeta., is the injection efficiency (which is a function
of cavity length scanning frequency), and T is the cavity
transmission. For example, a suitable laser can readily produce 20
mW of power at 488 nm e.g., an argon ion laser. For a 1 kHz
scanning frequency, the intracavity power will be approximately 1
watt. For a slower scanning frequency of 100 Hz, the intracavity
power will be approximately 9 watts. Given a mode size of 0.25 mm,
the intracavity power density is on the order of 500 watts/cm.sup.2
for a 1 kHz scanning frequency. These power densities are equal to
those found in commercial flow cytometers. The incident power
captured by a 1 .mu.m diameter particle will be approximately 4
.mu.W. By way of example, for a 1 .mu.m diameter fluorescein coated
polystyrene sphere, assuming a molar extinction coefficient of
90,000 M.sup.-1 cm.sup.-1, a single particle will absorb
approximately 30% of the incident light. With the intracavity power
estimated above, this leads to the generation of approximately 1
.mu.W of fluorescence emission per particle.
[0060] To determine the amount of light that will be captured by
the fluorescence detector, one first needs to calculate the amount
of emitted fluorescence that can be collected. For example, if one
uses a 0.17 NA (numerical aperture) lens with a magnification of
0.1, the viewing volume is 0.16 cm.sup.3. For a particle
concentration of 100 per cm.sup.3, there will be 3.2 particles in
the viewing region during each ringdown event if the collection
lens is placed in the center of the ringdown cell.
[0061] With a system viewing angle of 6 degrees, the corresponding
solid angle is: 3 = 4 sin ( 4 ) 2
[0062] or .OMEGA.=8.6.times.10.sup.-3 steradians. The fraction of
power collected by the optical system will be .OMEGA./4.pi. or
approximately 7.times.10.sup.-4. Thus the total power incident onto
the detector will be approximately 0.7 nW. For a PMT with a
sensitivity of 80 mA/watt, a gain of 1.times.10.sup.5, and a load
resistance of 1 M.OMEGA., this system should produce about 5.6 V of
signal per particle. The voltage noise of the PMT under these
conditions is less than 1 mV, which will result in a very high
signal to noise (SIN) ratio. Our system permits measurement of both
the total laser induced fluorescence and also the fluorescent
spectrum at the incident wavelength which significantly enhances
the discrimination between biological and non-biological
aerosols.
* * * * *